1. Field of the Invention
The present invention relates generally to digital modulators and baseband signal generators for digital modulators, and more particularly, to digital modulators used as MODEMs for digital communication equipments such as a land mobile radio telephone, a portable radio telephone and a cordless telephone, and baseband signal generators for such digital modulators.
2. Description of the Background Art
A conventional digital communication apparatus modulates a carrier signal in response to a digital information signal (baseband signal) to transmit the information signal in order to achieve efficient transmission.
Such modulation systems include an amplitude modulation system wherein an amplitude of a carrier signal is changed in response to a digital baseband signal (a modulating wave signal), a frequency modulating system, wherein a frequency of a carrier is deviated in response to a modulating wave signal, a phase modulating system wherein a phase of a carrier is changed in response to a modulating wave signal and an amplitude phase modulating system wherein an amplitude and a phase of a carrier are individually changed in response to a modulating wave signal.
The carrier signal (modulated signal) S(t) thus modulated in response to a modulating wave signal can be generally expressed by the following equations. ##EQU1##
Herein, A(t) denotes an amplitude, .omega.c denotes a carrier frequency and .phi.(t) denotes a phase of a modulating wave signal.
As is clear from the above-described equation (1), the modulated signal can be represented by two components orthogonal to each other, that is, by a sum of an in-phase (I phase) component (the first term of the above-described equation (1)) and a quadrature phase (Q phase) component (the second term of the above-described equation (1)). Such a modulated signal can be therefore formed by using a quadrature modulator.
FIGS. 1 and 2 are a block diagram and a spatial, diagram schematically showing the principle of such a quadrature modulator, respectively. It should be noted that the following example shows a phase modulating system for changing a phase of a carrier in response to a baseband signal, wherein an amplitude A (t) is fixed to "1".
With reference to FIG. 1, a mapping circuit 2 outputs I phase and Q phase components of a modulating wave signal as rectangular signals in response to a digital baseband signal applied through an input terminal 1. The I phase component is applied to one input of a multiplier 7 through a low pass filter (LPF) 3, while the Q phase component is applied to one input of a multiplier 8 through a low pass filter LPF 4.
A carrier signal cos.omega.ct is applied from a signal source 5 to the other input of the multiplier 7 which outputs an I phase component sin.phi.(t).multidot.cos.omega.ct of a modulated signal. A signal sin.omega.ct obtained by shifting the phase of the carrier signal from the signal source 5 by .pi./2 by means of a phase shift circuit 6 is applied to the other input of the multiplier 8 which outputs an Q phase component cos.phi.(t).multidot.sin.omega.ct of the modulated signal. Thus obtained I phase component and Q phase component can be represented in a one-to-one correspondence on the I and Q coordinates as shown in FIG. 2.
These I phase component and Q phase component are added to each other by an adder 9 to become such a modulated signal as expressed by equation (1), which signal is output from an output terminal 10.
FIG. 3 is a block diagram showing a GMSK (Gaussian filtered Minimum Shift Keying) modulator as an example of the quadrature modulator shown in FIG. 1. Such GMSK modulator is disclosed in, for example, "Differential Detection of GMSK Using Decision Feedback" by Abbas Yongacoglu et al., IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 36, No. 6, JUNE 1988, pp. 641-649 and "GMSK Modulation for Digital Mobile Radio Telephony" by Kazuaki Murota et al., IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM-29, No. 7, JULY 1981, pp. 1044-1050.
In FIG. 3, the portion 2 surrounded by the chain dotted line shows the details of the structure corresponding to the mapping circuit 2 and LPFs 3 and 4 of FIG. 1.
First, a digital baseband signal applied through the input terminal 1 is applied to a gaussian filter 14. More specifically, the gaussian filter 14 comprises a shift register 14a for taking the digital baseband signal by R bits and converting the same into R bit parallel data in response to a first clock signal applied from a clock signal source (not shown) through an input terminal 12 for every bit period T, a binary counter 14b for counting and outputting S bit data in response to a second clock signal applied from the clock signal source (not shown) through an input terminal 13 and having a frequency higher than that of the first clock signal, and a ROM type low pass filter 14c from which L bit data indicating a phase shift amount .DELTA..phi.(t) of a modulating wave signal is read, with the R bit output data of the shift register 14a as a higher order address and the S bit output data of the binary counter 14b as a lower order address.
The L bit data indicating the phase shift amount .DELTA..phi.(t) is applied to an integration circuit 15 including an adder 15a and a one-clock delay unit 15b. The integration circuit 15 integrates the applied phase shift amount .DELTA..phi.(t), outputs P bit data indicating the phase .phi.(t) of the modulating wave and applies the data to ROMs 16 and 17.
The ROM 16 includes a ROM table including I phase component data of the modulating wave signal and outputs the corresponding I phase component data of W bits in response to the data indicating the phase .phi.(t) from the integration circuit 15 as an address. The ROM 17 includes a ROM table including Q phase component data of the modulating wave signal and outputs the corresponding Q phase component data of W bits in response to the data indicating the phase .phi.(t) from the integration circuit 15 as an address.
The digital I phase component data output from the ROM 16 is converted into an analog I phase component signal sin.phi.(t) by a D/A converter 18 and applied to one input of the multiplier 7 through the LPF 3' in order to suppress sampling noises. The digital Q phase component data output from the ROM 17 is converted into an analog Q phase component signal cos.phi.(t) by a D/A converter 19 and applied to one input of the multiplier 8 through the LPF 4'. The subsequent operation is the same as previously described in connection with FIG. 1 and the terminal 10 outputs the modulated signal expressed by equation (1).
There is a case where M-phase PSK (Phase Shift Keying) signal is generated by using such a quadrature modulator. FIG. 4 is a diagram schematically showing the principle of the generation of .pi./4 shift QPSK (Quadli Phase Shift Keying) signal, which signal is one example of such a M-phase PSK signal.
With reference to FIG. 4, it is assumed that a signal point corresponding to I phase component and Q phase component data of a baseband signal (modulating wave signal) at a certain time point exists at one of a, c, e and g on the unit circle having a radius of 1 shown in FIG. 4. At a subsequent time point after a lapse of a predetermined time slot, the signal point shifts to one of the intersections b, d, f and h between two virtual axis obtained by rotating the I axis and the Q axis by .pi./4 and the unit circle of a radius of 1. The I axis and the Q axis will be rotated by .pi./4 for each predetermined time slot in the same manner as described above, whereby the signal point sequentially shifts on the unit circle.
For example, if the signal point initially exists at the point a in FIG. 4 and the baseband signal does not change, the signal point shifts as a point.fwdarw.b point.fwdarw.c point.fwdarw.d point.fwdarw.e point.fwdarw.f point.fwdarw.g point.fwdarw.h point for every predetermined time slot, that is, at every .pi./4 rotation of the I axis and the Q axis. In this case, the I and Q phase data each takes the five types of values such as "1", "1/.sqroot.2", "0", "-1/.sqroot.2 " and "-1" as can be seen from FIG. 4.
As described in the foregoing, in the modulation system in which each signal pattern of the I phase and the Q phase of a modulating wave signal has a plurality of levels, the use of a conventional quadrature modulator as shown in FIG. 3 employing ROM tables results in a drastic increase in the storage capacities of the ROMs 16 and 17, thereby increasing a manufacturing cost.
More specifically, where one ROM is provided for each of the I phase and the Q phase of the modulating wave signal as shown in FIG. 3, when each phase data has binary levels, one signal point, that is, one symbol requires one bit as an address of each ROM. When each phase data has five value levels, each symbol requires 3 bits as an address of each ROM. Furthermore, when each phase data has n-value levels, the number of bits of the address of each ROM required for each symbol is an integer equal to or more than log.sub.2 n. Therefore, with an increase in the number of levels of each phase data, the storage capacity of the ROM is increased in ogarithmic-functional manner.
In addition, when a single ROM is provided for each level of each data of the I phase and the Q phase to take a sum of the outputs of the respective ROM, each symbol requires one bit as an address of each ROM. When the number of levels of each data is n, the I phase and the Q phase require 2n number of ROMs in total.
As described in the foregoing, a quadrature modulator of a conventional structure used as a M phase PSK modulator such as a .pi./4 shift QPSK modulator requires a large capacity ROM or a large number of ROMs, which increases a manufacturing cost.
Such a conventional quadrature modulator as shown in FIG. 3 is structured such that the digital modulating wave component data (the outputs of the ROMs 16 and 17) are once converted into analog modulating wave component signals by the D/A converters 18 and 19, which are multiplied by carrier signals in an analog manner. Therefore, if the signal gains of the I phase component and the Q phase component of the modulating wave signal are different from each other in the stages subsequent to the LPFs 3' and 4' in FIG. 3, the spatial coordinates of the signal points are not located on the unit circle as in FIG. 2 and the signal locus becomes an ellipse. In such a case, the accurate modulating wave signal components can not be obtained and a satisfactory modulated signal can not be obtained accordingly.
Also in a case where the phases of two carrier components, which phases are shifted by .pi./2 from each other, are not precisely controlled, it is impossible to obtain a satisfactory modulated signal.
In addition, since numerous analog signal processing circuits are included in the prior art of FIG. 3, such circuit structure of the quadrature modulator as a whole can not be suitably made into an integration circuit.